Conventionally, an aircraft is provided with a flight management system or (FMS), configured for determining a reference trajectory of the aircraft from the flight plan.
In particular, the flight management system is configured for determining an a priori descent trajectory, from the cruising altitude of the aircraft as far as an interrupted approach point, depending on a descent slope, which is generally defined by default, and on the flight plan, notably by observing the altitude constraints of this flight plan.
Such a descent trajectory generally comprises, from the cruising altitude of the aircraft, a descent phase according to the predefined descent slope, from a point for changing level generally called a top of descent point (TOD), followed by a succession of segments each corresponding to a level flight or to a descent phase as far as the interrupted approach point (MAP for «Missed Approach Point»), while observing the points of constraints.
During the actual descent of the aircraft along the thereby determined descent trajectory, the aircraft is subject to additional constraints, notably to deck altitudes set by the air traffic control authority. In particular, a descent phase can only be initiated when an agreement with the air control authority has been obtained, i.e. if the authorized deck altitude for the aircraft is less than the current altitude of the aircraft.
Consequently, in certain circumstances, the aircraft may attain the top of descent point without having been authorized to carry out the descent phase provided by the a priori trajectory. Moreover, a descent phase provided by the a priori trajectory may be interrupted if the authorized deck altitude for the aircraft is greater than the final altitude of the descent phase.
Under such circumstances, as soon as the authorized deck altitude is lowered, the aircraft initiates a descent phase with a higher descent slope than the initially intended descent slope, with the purpose of joining with the trajectory determined a priori.